@article{SM_2011_202_11_a1,
     author = {A. A. Ardentov and Yu. L. Sachkov},
     title = {Extremal trajectories in a~nilpotent {sub-Riemannian} problem on the {Engel} group},
     journal = {Sbornik. Mathematics},
     pages = {1593--1615},
     publisher = {mathdoc},
     volume = {202},
     number = {11},
     year = {2011},
     language = {en},
     url = {http://geodesic.mathdoc.fr/item/SM_2011_202_11_a1/}
}